Organic boronic acids or ester compounds thereof are stable in water and air, and are utilized in cross-coupling reactions using a transition metal complex as a catalyst. Particularly, the reaction using a palladium compound as a catalyst is known as the Suzuki-Miyaura reaction, which is industrially used in pharmaceutical synthesis, pesticide synthesis, liquid crystal material synthesis, and the like. (see Non-Patent Document 1).
Moreover, recently, aromatic diboronic acids or ester compounds thereof are often used in the investigation study of OLED and conductive polymer materials. However, it is difficult to purify aromatic diboronic acids because many of them contain boroxine as an impurity. There is another drawback that aromatic diboronic acids or ester compounds are converted into boroxine by heating, thereby reducing the reactivity. In contrast, it is easy to purify aromatic diboronic acid ester compounds; however, many of them are poorly soluble in organic solvents and have a very high melting point.